Rotary screw machine with thrust balanced bearings

ABSTRACT

In a rotary screw machine, a shaft journal (7) of one of the rotors (6) is mounted in main thrust bearings (9, 10) and a balancing thrust bearing (11 ), the latter being located outermost on the shaft journal (7). The main thrust bearings (9, 10) are clamped towards the low pressure end of the machine. The balancing thrust bearing (11) is affected by a thrust balancing device for counter-acting the gas forces on the rotor (6), thereby reducing the force that has to be transferred by the main thrust bearing.

BACKGROUND OF THE INVENTION

The present invention relates to a rotary screw machine for a gaseousworking medium with at least one screw rotor operating in a casingincluding a low and a high pressure end section, which rotor atoperation is exposed to axial gas forces in a direction from the highpressure end section towards the low pressure end section, defined asthe positive axial direction. The rotor has at least one shaft journalmounted in beatings in one of said end sections, which beatings includethrust bearing means with main thrust bearing means adjacent the rotorand balancing thrust bearing means adjacent the outer end of the shaftjournal, which thrust bearing means are provided with thrust balancingmeans located axially between the main and balancing thrust bearingmeans and exerting a positively directed axial force on the main thrustbearing means and a negatively directed axial force on the balancingthrust beating means.

The term thrust bearing in this application is used to mean bearingswhich transfer at least axial loads, i.e. pure axial beatings as well asbearings transferring both axial and radial loads.

A rotary screw machine with such a thrust balancing arrangement isdisclosed in U.S. Pat. No. 4,915,514, which hereby is incorporated byreference. In that disclosure the general advantages gained by a thrustbalancing device are discussed as well as the particular advantagesattained by the above specified kind of thrust balancing, namely longerrunning life for the thrust bearings and a simplified exchange ofbearings.

In that known device, the main thrust beating means transfer axialforces in the positive direction only whereas the balancing thrustbeating means transfer axial forces in both directions. A preloadingspring acts on the balancing thrust bearing means, securing a properaxial localisation of the rotor so that a certain play between the highpressure end of the rotor and the adjacent end wall is assured when themachine is at rest or idles. At operation the positively directed axialgas Forces on the rotor are partly balanced by a piston of the thrustbalancing means acting on the outer ring of the balancing thrust bearingmeans in the negative axial direction. The balancing force is mainlyreceived in that gas from the high pressure side of the machine isconducted to the piston to act thereon. The balancing force will belarger than the force from the preloading spring with the effect thatthe resulting force on the outer ring of the balancing thrust bearingmeans is negatively directed, which force is transmitted to the rotorand counteracts the axial gas forces. The main thrust bearing meanstherefore need to transfer only the remaining part of the axial gasforces.

This known device functions satisfactory for applications where thedischarge pressure (when the machine is a compressor) is above a certainlevel, in which case it is possible to dimension the piston large enoughto attain a force which is larger than the preload force. The availablespace for the piston, however, is limited, which in cases when thedischarge pressure is not so high will raise problems to attain abalancing force exceeding the preloading force, which has to be above acertain level. In such cases, e.g. when the machine is a refrigerationcompressor using refrigerant of the type R134a, the known devicetherefore cannot function satisfactory.

SUMMARY OF THE INVENTION

The object of the present invention is to improve the known thrustbalancing device so that it is capable of functioning also at lowerpressure levels at its high pressure side.

According to the invention this has been achieved in that there isprovided clamping means acting on the main thrust bearing means in thepositive direction and the main thrust bearing means are capable oftransferring axial forces in both directions and the balancing thrustbearing means are capable of only transferring forces which act on theshaft in the positive direction.

Since according to the invention, there is no preloading in the positivedirection on the balancing thrust bearing means, it is attained that theforce from the thrust balancing means on the thrust balancing bearingmeans is entirely available for the balancing without the need to firstcompensate a preloading force. The desired effect thus can be achievedalso with lower forces for the actuation of the thrust balancing means.

The solution according to the invention also has the advantage that thebalancing thrust bearing means also can be of a cheaper kind. Sincethese bearings are exchanged at shorter intervals than the main thrustbearing means the total bearing costs will be reduced.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will be further explained through the following detaileddescription of a preferred embodiment thereof and with reference to theaccompanying drawings.

FIG. 1 is a schematic side view of a rotary screw compressor.

FIG. 2 is a section through a shaft journal of a rotor in a rotary screwcompressor according to the invention.

DETAILED DESCRIPTION

The rotary screw compressor illustrated in FIG. 1 has a pair ofintermeshing screw rotors 6 operating in a casing I having a lowpressure end section 2 and a high pressure end section 3. The workingmedium, e.g. refrigerant R134a enters the compressor through an inletchannel 4 and after being compressed leaves the compressor through anoutlet channel 5. Each rotor is mounted in bearings in both end sections2, 3. The pressure difference between the low and high pressure ends ofthe compressor during operation results in that the gas exerts an axialforce on each rotor from the high pressure end towards the low pressureend. This is defined as the positive axial direction.

FIG. 2 is a section through the shaft journal 7 of one of the rotors inthe high pressure end section 3, and the arrow F_(A) represents thepositively directed axial force on the rotor. The shaft journal 7 ismounted in a plurality of bearings, namely a radial roller bearing 8 andthrust ball bearings 9, 10 and 11 as seen in order from the rotor to theend of the shaft journal 7. The two inner thrust ball bearings 9, 10constitute the main thrust bearings and each of them is of the kindcapable of transferring axial forces in only one direction. One ismounted to transfer forces in the positive direction and the other onein the negative direction so that they taken together are capable oftransferring forces in both directions. The thrust ball bearing 11 atthe outer end of the shaft journal is a thrust balancing bearing and isseparated from the main thrust bearings 9, 10 by a distance sleeve 21.The thrust balance bearing is capable of transferring axial forcesacting on the shaft journal 7 in the positive direction only. All thebearings 8, 9, 10, 11 are kept in place by a key disc 25 at the end ofthe shaft journal 7.

Axially between the main thrust bearings 9, 10 and the balancing thrustbearing 11 a thrust balancing device 13 is provided, which consists oftwo annular members 18, 19 defining a sealed annular chamber 23 betweenthem. The annular chamber 23 communicates with a pressure medium channel22 establishing fluid communication with the working fluid at thedischarge side of the compressor. The annular chamber accommodates athrust spring 15 which holds the annular members 18 and 19 apart andpreloads the thrust balancing bearing 11.

A cup spring 12 clamps the outer rings of the main thrust ball bearings9, 10, the clamping force being transmitted by a sleeve 20 and theannular member 18 of the thrust balancing device 13. Alternatively thesleeve 20 and the annular member can be made as a single unit. By theclamping force it is secured that the rotor will be axially fixed atrest or when the compressor idles, and thus the rotor I cannot come intocontact with the adjacent high pressure end wall.

When loaded the discharge pressure is transmitted to the annular chamber23 through the pressure medium channel 22, and the pressure acting onthe inner surface 24 of the annular member 19 exerts a negativelydirected force on the outer ring 17 of the balancing thrust bearing 11,which force thus counteracts the gas force F A therewith reducing theaxial force that has to be transferred by the thrust ball bearing 9 ofthe main thrust bearing means.

As should be obvious to a person skilled in the art variousmodifications can be made without departing from the scope of theinvention. For example the two bearings of the main thrust bearing meanscould be replaced by a double-acting thrust bearing and each bearingcould of course be replaced by two or more bearings performing the samefunction.

I claim:
 1. A rotary screw machine having a low pressure side and a highpressure side, for use with a gaseous working medium, comprising:acasing (1) having a low pressure end section (2) and a high pressure endsection (3); at least one screw rotor (6) operating in said casing, saidat least one rotor (6) at operation being exposed to axial gas forces(F_(A)) in a positive axial direction from said high pressure endsection (3) towards said low pressure end section (2), and said at leastone rotor (6) having at least one shaft journal (7) mounted in bearings(8, 9, 10, 11) in one of said end sections of said casing (1), said atleast one shaft journal (7) having an outer end; said bearings (8, 9,10, 11) including thrust bearing means (9, 10, 11) with main thrustbearing means (9, 10) adjacent to said at least one rotor (6) andbalancing thrust bearing means (11) adjacent said outer end of said atleast one shaft journal (7); said thrust bearing means (9, 10, 11)including thrust balancing means (13) located axially between said mainthrust bearing means (9, 10) and said balancing thrust bearing means(11) and exerting a positively directed axial force on said main thrustbearing means (9, 10) in said positive axial direction, and a negativelydirected axial force on said balancing thrust bearing means (11) in anegative axial direction which is opposite to said positive axialdirection; clamping means (12) acting on said thrust bearing means (9,10, 11) in said positive axial direction, said clamping means (12)acting only on said main thrust bearing means (9, 10) of said thrustbearing means (9, 10, 11); and said main thrust bearing means (9, 10)transferring axial forces in both of said positive and negative axialdirections, and said balancing thrust bearing means (11) onlytransferring forces which act on said shaft journal (7) in said positiveaxial direction.
 2. The rotary screw machine of claim 1, wherein saidthrust balancing means (13) includes spring means (15).
 3. The rotaryscrew machine of claim 2, wherein said thrust balancing means (13)further includes fluid-pressure actuated means (18, 19).
 4. The rotaryscrew machine of claim 3, wherein said fluid-pressure actuated meansincludes two annular members (18, 19) which are axially expandable andform an annular pressure chamber (23) between each other, said annularpressure chamber (23) being in communication with a gas channel (22)establishing a fluid-connection between said annular pressure chamber(23) and said high pressure side of the rotary screw machine.
 5. Therotary screw machine of claim 1, wherein said clamping means (12)comprises a cup spring.
 6. The rotary screw machine of claim 1,wherein:said main thrust bearing means comprises two single-actingthrust bearings (9, 10) transferring forces in opposite directions; andsaid balancing thrust bearing means comprises only one single-actingthrust bearing (11).
 7. The rotary screw machine of claim 2, whereinsaid clamping means (12) comprises a cup spring.
 8. The rotary screwmachine of claim 2, wherein:said main thrust bearing means comprises twosingle-acting thrust bearings (9, 10) transferring forces in oppositedirections; and said balancing thrust bearing means comprises only onesingle-acting thrust bearing (11).
 9. The rotary screw machine of claim3, wherein said clamping means (12) comprises a cup spring.
 10. Therotary screw machine of claim 3, wherein:said main thrust bearing meanscomprises two single-acting thrust bearings (9, 10) transferring forcesin opposite directions; and said balancing thrust bearing meanscomprises only one single-acting thrust bearing (11).
 11. The rotaryscrew machine of claim 4, wherein said clamping means (12) comprises acup spring.
 12. The rotary screw machine of claim 4, wherein:said mainthrust bearing means comprises two single-acting thrust bearings (9, 10)transferring forces in opposite directions; and said balancing thrustbearing means comprises only one single-acting thrust bearing (11).